This invention relates to improvements in a spectrophotometer of the single wavelength--double beam type or the dual wavelength type employing the dynode feedback system.
In the spectrophotometer of the single wavelength--double beam type, the ratio of absorptions at any desired single wavelength between two cells, i.e., a sample cell and a reference cell, is measured. More specifically, monochromatic light of a single wavelength derived from a light source is chopped by a rotating sector mirror and a chopper into a sequence of light signals comprising sample light and reference light having predetermined periods, which signals alternately enter a sample cell and a reference cell situated in a sample chamber. The light signals from the sample cell and the reference cell are detected and converted into electric signals in the form of a sample signal and a reference signal, respectively, by means of a photomultiplier. On the basis of the sample signal and the reference signal thus obtained, the absorption and transmission factor of the sample are measured.
On the other hand, in the spectrophotometer of the dual wavelength type, the difference in the degree of absorption by a sample between two signals of different wavelength is measured for one sample cell. More specifically, light from a light source is separated by a monochromator into a light signal having a wavelength serving as the reference of the spectrum absorption of the sample to be measured and a light signal having any other desired wavelength. The light signals are caused to alternately enter the single sample cell by means of a chopper, and the resultant signals from the sample cell for the respective wavelengths are detected and converted into electrical signals by means of a photomultiplier. The difference in the degree of absorption at the two wavelengths thus obtained is measured.
With respect to the function of the photomultiplier, however, it may be considered in both the single wavelength--double beam type and the dual wavelength type that the one beam which is caused to enter in order to measure the absorption or transmission factor of the sample is the reference light serving as the reference, while the other beam is the sample light to be measured.
In spectrophotometers of these types, the photomultiplier is used as the detector, as stated previously. In the photomultiplier, a large number of electrodes called "dynodes" for emitting secondary electrons are disposed between an anode and a cathode. The photomultiplier accordingly has the characteristic that a linear relationship holds between the logarithmic indication of voltages applied thereto and the logarithmic indication of output currents thereof.
As an expedient for automatically controlling the applied voltage in consideration of such characteristics of the photomultiplier, there is the so-called dynode feedback method. The dynode feedback method will be explained by taking as an example the foregoing spectrophotometer of the single wavelength--double beam type. The light from the light source is chopped by the rotating sector mirror and the chopper into the sample light and the reference light, which alternately enter the sample cell and the reference cell. The light signals from the sample cell and the reference cell are respectively converted into the electric signals which comprise the sample signal and the reference signal by the photomultiplier. The signals from the photomultiplier are amplified by an amplifier. Thereafter, the voltage corresponding to the reference signal above is compared with a reference voltage in a differential amplifier, and the difference is supplied to a DC/DC converter. While the DC/DC converter applies a fixed voltage to the photomultiplier, it adjusts the applied voltage of the photomultiplier in accordance with the differential voltage from the differential amplifier and controls the reference signal from the photomultiplier so that it is constant at all times.
In this manner, the feedback control is made so as to render the reference signal constant, and the absorption and transmission factor of the sample are measured from the sample signal at that time.
In the spectrophotometer employing the dynode feedback method, the sample signal and the reference signal are alternately detected by the photomultiplier. When the latter signal is used to control the feedback, the applied voltage of the photomultiplier changes gradually during the detection of the sample signal. This leads to the problem that the stability of the feedback system is inferior. More specifically, when a signal processing system for the sample signal and the reference signal is provided by analog circuitry, the applied voltage of the photomultiplier changes in accordance with the time constant of the system. On the other hand, when the signal processing system is in the form of digital circuitry employing a counter, the applied voltage changes in accordance with the integration characteristics of the counter.